Today, let us blow hot and cold. The University of
Houston's College of Engineering presents this
series about the machines that make our
civilization run, and the people whose ingenuity
created them.

An old children's story tells
of a traveler lost in a vast forest one winter's
night. He stumbles into a widow's hut and begs for
a bowl of soup by her fire. The woman says, "Yes."

He stands blowing on his hand while she ladles the
soup. "What are you doing?" she asks. "Why, my
hands are cold. I'm warming them with my breath."
She eyes him suspiciously as she hands him his
soup. He sits down with the bowl, and blows across
the spoon before he puts it in his mouth.

"Now what are you doing," she cries. He glances up,
surprised, and says, politely, "The soup is so
wonderfully hot. I simply mean to cool it before I
try to swallow it." The woman seizes a fire-iron
and shouts, "Get out! Get out of my house! I'll
have no sorcerer who can blow both hot and cold
under my roof!"

Interesting story: Perhaps it was meant to tell
children to be consistent -- something we need to
think about. But, before we do, let's look at the
literal act of blowing hot and cold

Air leaves our body at a little over 98 degrees
Fahrenheit. When we come in out of the cold, we
open our mouth wide and exhale that warm air upon
our hands. We clear the air passage so the warm air
leaves almost unimpeded.

Cooling soup is another matter. This time we purse
our lips and build up air pressure in our mouth.
Our lips are now a nozzle through which air leaves
at substantial speed. That air jet is now a bit
cooler than it was in our mouth, but another cooling
effect is dominant. The jet entrains a great deal
of cool room air, which flows over the soup
convectively cooling it.

So there is no sorcery. We all really do blow both
hot and cold, and we do it instinctively. Yet
there's that story. And another like it tells
of a king who grew frustrated with advisors who
kept telling him, "On the other hand..." Finally he
shouted to his chamberlain, "Go out and find me a
one-armed advisor."

Something there is that does not like ambiguity.
Yet reality shifts under our feet. New technology
constantly recreates expectation by making a
different place of our world. So what about blowing
hot and cold? The Bible tells us that, "Because you
are lukewarm -- neither hot nor cold -- I will spew
you from my mouth."

Does that mean we should all run around blowing hot and cold?
I doubt it. Rather, we cannot let timidity drive us to
blow neither hot nor cold. A Wright Brothers biographer
tells how Orville and Wilbur argued so ferociously that you
had to watch closely -- because they often
exchanged positions during their verbal combat.

Maybe it's useful to dislike uncertainty. For only by embracing
an idea fully can we really test and reject it -- as the Wright
Brothers did. Perhaps, if I can paraphrase Barry Goldwater,
some extremism, some heat and cold, may not be entirely a vice.
Well, as long as it's part of our ongoing pursuit of truth.

I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.

(Theme music)

Click here
for a useful article on jet entrainment. The images in this article reveal
the huge amount of room air (or water in the case of a water jet) that is
entrained into the jet.

The secondary effect is air cooling by adiabatic depressurization as it leaves the
mouth, see, e.g., W. C. Reynolds and H. C. Perkins, Thermodynamics.
(New York: McGraw-Hill Book Co., 1977): especially, pg. 249, eqn. 8.33. The
temperature of exiting air is Te = Ti(pe/pi)(k-1)/k,
where i denotes conditions outside the mouth, e denotes conditions within the mouth,
T is the absolute temperature, and p is the absolute pressure. The constant k is
the adiabatic constant, approximately 1.41 for air. This means that the absolute
temperature varies as p0.29. The slight pressure difference can change the air
temperature by only a few degrees.